1. Field of the invention
The present invention relates to a process for working up crude liquid vinyl acetate which contains acetic acid and water and also ethyl acetate and, if appropriate, further impurities.
2. Discussion of prior art
Processes for the manufacture of vinyl acetate frequently yield a liquid crude vinyl acetate which essentially contains acetic acid and water and also ethyl acetate and, if appropriate, further impurities. A product of this type can be obtained, for example, in the manufacture of vinyl acetate by reacting ethylene with acetic acid and oxygen, in the gas phase, at elevated temperature and under normal or elevated pressure, in the presence of a catalyst, the gases leaving the reactor being cooled and/or washed with acetic acid. This type of manufacture of vinyl acetate is generally carried out with excess acetic acid, the unreacted acetic acid being recycled into the vinyl acetate manufacture.
A method of working up crude liquid vinyl acetate, accessible by such manufacturing processes, which still contains acetic acid and water and also ethyl acetate and, if appropriate, further impurities, is described in German Auslegeschrift 1,768,412. In this process, the crude vinyl acetate is azeotropically distilled, the major part of the water, together with vinyl acetate, is drawn off at the top by recycling the vinyl acetate obtainable by condensation and phase separation of the top product, and vinyl acetate is recycled in an amount such that an acetic acid which contains about 0.5 to 6% by weight of water is obtained as the bottom product. If appropriate, a liquid sidestream, in which ethyl acetate is concentrated, can be removed from this distillation. In this process, the water content of the top vapours from the distillation is about 3.4% by weight (compare the repetition of the process of German Auslegeschrift 1,768,412. carried out according to Example 1). The vinyl acetate obtainable after condensation and phase separation of the gaseous top product contains about 200 to 500 ppm by weight of ethyl acetate.
Another known process for working up crude vinyl acetate is described in German Pat. No. 1,282,014 and German Pat. No. 1,668,063. In this process, the water, as an azeotropic mixture, and the by-products boiling at a lower temperature than vinyl acetate are distilled overhead, and the virtually anhydrous bottom product is distilled in a second distillation column, vinyl acetate being removed at the top of the second distillation column, at least the major part of the by-products boiling at a higher temperature than vinyl acetate being removed from one or more concentration zones between the top and the bottom, and the acetic acid and, if appropriate, the remainder of the by-products being removed below the lowest concentration zone or as the bottom product. The vinyl acetate obtained in the first column after condensation and phase separation is generally recycled completely into the first column, and vinyl acetate is only removed from the top of the second column. No information on the water content of the gaseous top product of the first column is given, and the ethyl acetate content in the vinyl acetate separated off is about 1,000 ppm.
German Offenlegungsschrift 2,943,985 describes a process for the separation of water from mixtures with vinyl acetate and acetic acid, in which two different mixtures of crude vinyl acetate are introduced into a distillation column at various points. The top product can then contain about 3 to 5% by weight of water. In cannot be inferred from this German Offenlegungsschrift whether crude vinyl acetate containing ethyl acetate can be worked up in this way and what contents of ethyl acetate are then present in the vinyl acetate separated off.
Finally, German Auslegeschrift 1618240 describes a method for separation of ethyl acetate from vinyl acetate, in which extractive distillation is carried out using water as the entrainer. In this process, however, the vinyl acetate used does not contain significant amounts of water and/or acetic acid.